This invention relates to thin film circuit fabrication, and in particular to a method for fabricating resistors and capacitors on a single substrate.
Fabrication of thin film resistors and capacitors on a single insulating substrate has been known for many years. Typically, the resistors comprise a layer of tantalum nitride which is deposited by sputtering, followed by chemical etching to form the desired pattern. The capacitors are typically formed by depositing a layer of tantalum which is etched to define the anode, followed by anodization of a portion of the layer to form the capacitor dielectric. The capacitor counter-electrode is then formed over the dielectric by depositing and patterning layers which are typically nickel-chromium and gold. These steps are performed in a variety of combinations in order to achieve maximum processing efficiency and compatibility between the components. (See, e.g. U.S. Pat. No. 3,607,679, issued to Melroy et al and U.S. Pat. No. 3,718,565, issued to Pelletier.)
At some point in the processing, it is necessary to stabilize the resistor films by heating in an oxidizing atmosphere. In prior art processes, this step was usually performed prior to completion of the capacitors, otherwise the capacitor dielectric would be degraded resulting in excessive leakage currents. While prior art processes were adequate, the stabilization problem in part necessitated a departure from an optimized sequence which would involve formation of all resistors and capacitors followed by stabilization. In cases where stabilization was prescribed after all resistors and capacitors were formed, the capacitor dielectric had to be formed with a high voltage anodization to insure no degradation (see, U.S. Pat. No. 4,251,326, issued to F. R. Arcidiacono, et al.
The problem of stabilization has recently been exacerbated by the need to further miniaturize thin film components to conserve space. This is especially important where thin film resistors and capacitors are formed over a silicon substrate including active silicon devices. In such circuits, capacitors should be anodized at low voltages less than 100 volts to form a very thin dielectric (less than 1700 Angstroms) in order to achieve high capacitance density. Such thin dielectrics cannot stand up to the high temperatures and times required to stabilize the resistors (typically 350 degrees C. for one hour). However, it appears that the fabrication technique may not be economically attractive unless both the resistors and capacitors can be formed completely prior to stabilization.
It is therefore an object of the present invention to provide a stabilization process which would permit prior complete formation of resistors and capacitors so that processing is optimized. It is a further object of the invention to provide a stabilization process which is compatible with both thin film resistors and low voltage anodization thin film capacitors.